Optical scanning device decreasing amount of change in relative scanning position

ABSTRACT

An optical scanning device includes a plurality of scanning optical systems configured to scan different scanning surfaces. Each of the scanning optical systems includes a light source configured to emit a light flux, a deflector configured to scan the light flux emitted from the light source, wherein the deflector is commonly used in the plurality of scanning optical systems. Each of the scanning optical systems further includes a scanning lens configured to condense the scanned light flux to the scanning surface, an optical path inflection mirror configured to inflect the scanned light flux, and an imaging lens configured to lead the light flux emitted from the light source to the deflector. The plurality of scanning optical systems are provided at both sides of the deflector having the deflector therebetween such that one each of the scanning optical systems at both sides of the deflector include a set of the optical scanning system and respective light fluxes scanned by the deflector in the set of the optical scanning system become approximately parallel in a main scanning direction, and an expression, |N−M|=2k+1 is satisfied when the number of optical path inflection mirrors provided in each of the set of scanning optical systems is represented by “N” (i.e., N≧2) and “M” (i.e., M≧1), and “k” is an integer equal to zero or larger.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning device and an imageforming apparatus using the optical scanning device, and moreparticularly to the optical scanning device in which an amount of changein a relative scanning position of each scanning optical system, whichis caused by a temperature fluctuation, is decreased.

2. Discussion of the Background

An image forming apparatus, in which a plurality of photoconductiveelements are scanned by single optical scanning device is commonlyknown. The image forming apparatus includes, for example, a colorprinter having a plurality of photoconductive drums, a color copierhaving a plurality of photoconductive drums, a high speed laser printer,a digital copier and so forth. In the above-described image formingapparatuses, a high speed print of a color image is performed becauseindividual photoconductive elements corresponding to each color areprovided. In addition, the number of parts used including a deflectorand a consumption of an electric power are decreased because a singleoptical scanning device is commonly used for scanning the plurality ofphotoconductive elements.

In the above type of image forming apparatus, a lens made of a resin,which has a positive power only in a sub-scanning direction, is used asan imaging lens to form a linear image in the vicinity of a deflectingsurface of a deflector. Hereinafter, a direction in which beam lightemitted from a light source is deflected by the deflector is referred toas a main scanning direction. A direction perpendicular to the mainscanning direction and in which a transfer sheet is conveyed is referredto as a sub-scanning direction. As the imaging lens, a lens made of aresin is commonly used instead of a lens made of a glass to reduce costsfor a material itself and processing of the material.

However, because the resin lens has a high linear expansion coefficient,an optical axis of the lens changes in a sub-scanning direction when asurrounding temperature fluctuates. Thus, a scanning position on asurface of a photoconductive element changes in the sub-scanningdirection. If a direction of change of the scanning position in thesub-scanning direction is different on each photoconductive element, acolor shift is created, resulting in a degradation of a produced image.

In Japanese Patent Laid-Open Publication No. 10-26732, an opticalscanning device and an image forming apparatus using the opticalscanning device in which a hybrid cylinder lens, which is integrallyproduced with a resin lens having a negative power in the sub-scanningdirection and a glass lens having a positive power in the sub-scanningdirection are disclosed. The hybrid cylinder lens is employed to reducean image surface bulge fluctuation caused by the temperaturefluctuation. However, the image surface bulge fluctuation in thesub-scanning direction is caused by the temperature fluctuation even ifthe hybrid cylinder lens is employed because a resin has a linearexpansion coefficient higher than that of a glass.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentioned andother problems and addresses the above-discussed and other problems.

The present invention advantageously provides a novel optical scanningdevice and an image forming apparatus using the optical scanning opticaldevice in which an amount of change in a relative scanning position ofeach scanning optical. system is decreased by aligning a direction ofchange of the scanning position in a sub-scanning direction caused by atemperature fluctuation in a plurality of scanning optical systems.

According to an example of the present invention, the optical scanningdevice includes a plurality of scanning optical systems configured toscan different scanning surfaces. Each of the scanning optical systemsincludes a light source configured to emit a light flux, a deflectorconfigured to scan the light flux emitted from the light source, whereinthe deflector is commonly used in the plurality of scanning opticalsystems. Each of the scanning optical systems further includes ascanning lens configured to condense the scanned light flux to thescanning surface, an optical path inflection mirror configured toinflect the scanned light flux, and an imaging lens configured to leadthe light flux emitted from the light source to the deflector.

Scanning optical systems are provided at both sides of the deflector,that is, the deflector is between a pair of scanning optical systems.Each of the scanning optical systems comprises a set of an opticalscanning system and respective light fluxes scanned by the deflector,the set of the optical scanning system being approximately parallel in amain scanning direction. The expression, |N−M|=2k+1 is satisfied whenthe number of optical path inflection mirrors provided in each of theset of scanning optical systems is represented by “N” (i.e., N≧2) and“M” (i.e., M≧1), and “k” is an integer equal to zero (0) or larger.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a diagram illustrating a layout of an optical system of anoptical scanning device in a plane parallel to a rotating plane of adeflector;

FIG. 2 is a diagram illustrating a sectional view of a scanning opticalsystem in the sub-scanning direction arranged after the deflector;

FIG. 3 is a diagram illustrating a sectional view of an example of thescanning optical system in the sub-scanning direction arranged beforethe deflector;

FIG. 4 is a diagram illustrating a sectional view of another example ofthe scanning optical system in the sub-scanning direction arrangedbefore the deflector;

FIG. 5 is a diagram illustrating a sectional view of another example ofthe scanning optical system in the sub-scanning direction arrangedbefore the deflector;

FIG. 6 is a diagram illustrating a sectional view of another example ofthe scanning optical systems in the sub-scanning direction arrangedbefore the deflector;

FIG. 7 is a diagram illustrating a sectional view of the scanningoptical system in the sub-scanning direction showing a change of anoptical axis due to a temperature fluctuation;

FIG. 8 is a diagram illustrating a sectional view of another example ofthe scanning optical systems in the sub-scanning direction arrangedbefore the deflector;

FIG. 9 is a diagram illustrating a sectional view of the scanningoptical system in the sub-scanning direction after the deflector; and

FIG. 10 is a schematic drawing illustrating a construction of a tandemimage forming apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, anoptical scanning device and an image forming apparatus according to anexample of the present invention are described below referring tofigures. FIG. 1 is a diagram illustrating a layout of an optical systemof an optical scanning device in a plane parallel to a rotating plane ofa deflector. The optical scanning device includes four scanning opticalsystems A, A′, B, and B′. Reference numerals 1A, 1A′, 1B, 1B′, and 2A,2A′, 2B, 2B′ denote a semiconductor laser as a light source, andcoupling lens, respectively. Reference numerals 3A, 3A′, 3B, 3B′, and 4denote an imaging lens made of a resin (hereinafter referred to as aresin lens), and a deflector, respectively.

In addition, reference numerals 5A, 5A′, 5B, 5B′, and 6A, 6A′, 6B, 6B′represent a first and second scanning lenses, respectively. Referencenumerals 7A, 7A′, 7B, 7B′ represent a soundproofing glass. The scanningoptical systems A, A′, B, B′ include respective above-described opticalelements. However, the deflector 4 is the element that is common to thescanning optical systems A, A′, B, B′. The deflector 4 includes, forexample, a polygon mirror and rotates at a constant speed in a directionindicated by an arrow in FIG. 1. Reference numerals 8A, 8A′, 8B, 8B′,and “ma”, “mb” denote a photoconductive element (i.e., a surface ofwhich is scanned), and a mirror, respectively.

The mirror “ma” inflects a light flux emitted from the semiconductorlaser 1A′ to the deflector 4. The mirror “mb” leads a light flux emittedfrom the semiconductor laser 1B′ to the deflector 4. A light fluxemitted from the semiconductor laser 1A is led to the deflector 4without being inflected by the mirror “ma”. A light flux emitted fromthe semiconductor laser 1B is led to the deflector 4 without beinginflected by the mirror “mb”. An optical path inflection mirror providedafter the deflector 4 is not shown in FIG. 1. In the example illustratedin FIG. 1, one light source is provided for each scanning opticalsystem. However, a plurality of light sources may be employed for eachscanning optical system.

The scanning optical systems A and B are a set of scanning opticalsystem and are provided at both sides of the deflector 4 having thedeflector 4 therebetween such that light fluxes scanned by the deflector4 become approximately parallel in a main scanning direction. Namely,the light fluxes emitted from the semiconductor lasers 1A and 1B, whichare positioned above the deflector 4 in FIG. 1, are reflected by thedeflector 4 toward the right side and the left side in FIG. 1 in thescanning optical systems A and B, respectively. Thus, the light fluxesscanning a surface of the photoconductive elements 8A and 8B becomeapproximately parallel in the main scanning direction.

Hereinafter, a direction in which the light fluxes emitted from thesemiconductor lasers 1A, 1A′, 1B, and 1B′ (i.e., light sources) aredeflected by the deflector 4 is referred to as the main scanningdirection. The direction which is orthogonal to the main scanningdirection is referred to as the sub-scanning direction. In FIG. 1, theset of scanning optical system includes a combination of the scanningoptical systems A and B′, A′ and B, A′ and B′ besides theabove-described combination of the scanning optical systems A and B.

The scanning optical systems A and A′ are arranged in the sub-scanningdirection having a distance therebetween. Similarly the scanning opticalsystems B and B′ are arranged in the sub-scanning direction having adistance therebetween. Namely, scanning optical systems are arranged atboth sides of the deflector 4 having the deflector 4 therebetween and aplurality of scanning optical systems are arranged in the sub-scanningdirection at both sides of the deflector 4.

Each diverging light flux emitted from each semiconductor laser 1A, 1A′,1B, and 1B′ is coupled by the respective coupling lenses 2A, 2A′, 2B,and 2B′. The light flux passed the respective coupling lenses 2A, 2A′,2B, and 2B′ then passes respective resin imaging lenses 3A, 3A′, 3B, and3B′, which have a positive power only in the sub-scanning direction, toform a linear image in the vicinity of a deflecting surface of thedeflectors 4.

FIG. 2 is a diagram illustrating a sectional view of the scanningoptical system in the sub-scanning direction arranged after thedeflector 4. Each light flux deflected by the deflector 4 is led to thephotoconductive elements 8A, 8A′, 8B, and 8B′ after passing the firstscanning lenses 5A, 5A′, 5B, and 5B′, the second scanning lenses 6A,6A′, 6B, and 6B′, and the soundproofing glasses 7A, 7A′, 7B, and 7B′ soas to form an image as a light spot on the surface of the respectivephotoconductive elements 8A, 8A′, 8B, and 8B′ while scanning the surfacethereof approximately at the same speed. An optical path inflectionmirror is provided in a light flux path between the first and secondscanning lenses in each scanning optical system A, A′, B, and B′. Theoptical path inflection mirror leads each light flux deflected by thedeflector 4 to the respective photoconductive elements 8A, 8A′, 8B, and8B′.

An arrangement of the optical path inflection mirror in each scanningoptical system A, A′, B, and B′ is described below. One piece of opticalpath inflection mirror Ma1 is provided in the scanning optical system Ato reflect the light flux from the first scanning lens 5A approximatelyat the right angle toward the lower direction in FIG. 2. Three opticalpath inflection mirrors Ma′1, Ma′2, and Ma′3 are provided in thescanning optical system A′. The optical path inflection mirror Ma′1reflects the light flux from the first scanning lens 5A′ approximatelyat the right angle toward the lower direction in FIG. 2. The opticalpath inflection mirror Ma′2 reflects the light flux from the opticalpath inflection mirror Ma′1 approximately at the right angle toward theleft in FIG. 2. The optical path inflection mirror Ma′3 reflects thelight flux from the optical path inflection mirror Ma′2 approximately atthe right angle toward the lower direction in FIG. 2.

Two optical path inflection mirrors Mb1 and Mb2 are provided in thescanning optical system B. The optical path. inflection mirrors Mb1reflects the light flux from the first scanning lens 5B toward a lowerright direction in FIG. 2. The optical path inflection mirror Mb2reflects the light flux from the optical path inflection mirror Mb1toward the lower direction in FIG. 2. Two optical path inflectionmirrors Mb′1 and Mb′2 are provided in the scanning optical system B′.The optical path inflection mirrors Mb′1 reflects the light flux fromthe first scanning lens 5B′ toward the lower left direction in FIG. 2.The optical path inflection mirror Mb′2 reflects the light flux from theoptical path inflection mirror Mb′1 toward the lower direction in FIG.2. The optical path inflection mirror in each scanning optical system isnot necessarily positioned between the first and second scanning lensesbut may be arranged at any position between the deflector 4 and therespective photoconductive element 8A, 8A′, 8B and 8B′.

FIG. 3 is a diagram illustrating a sectional view of an example of thescanning optical system in the sub-scanning direction arranged beforethe deflector 4. Reference numerals 1, 2, and 3 denote the semiconductorlaser, the coupling lens, and the resin imaging lens having a positivepower in the sub-scanning direction to form a linear image,respectively. Reference numerals 11, 14, and 14C represent a lightsource unit pedestal, a housing, and a supporting surface, respectively.Reference numerals 12 and 13 denote adhesives.

The semiconductor laser 1 is force-fitted and fixed to the light sourceunit pedestal 11. The coupling lens 2 is provided to the light sourceunit pedestal 11 with the adhesive 12 after a position of the couplinglens 2 is adjusted. The light source unit pedestal 11 and resin imaginglens 3 are provided to the housing 14. The resin imaging lens 3 isprovided to the housing 14 with the adhesive 13.

FIG. 4 is a diagram illustrating a sectional view of another example ofthe scanning optical system in the sub-scanning direction arrangedbefore the deflector 4. The resin imaging lens 3 is provided to thehousing 14 while being pressed from an upper end portion thereof towardthe supporting surface 14C by a flat spring 15.

FIG. 5 is a diagram illustrating a sectional view of another example ofthe scanning optical system in the sub-scanning direction arrangedbefore the deflector 4. The resin imaging lens 3 is force-fitted to thehousing 14 making the supporting surface 14C as a datum level.

FIG. 6 is a diagram illustrating a sectional view of another example ofthe scanning optical system in the sub-scanning direction arrangedbefore the deflector 4. The resin imaging lens 3 is provided to thehousing 14 with the adhesive 13. The adhesive 13 is applied to aposition adjacent to the optical axis of the resin imaging lens 3 unlikethe example illustrated in FIG. 3.

In a conventional optical scanning device, a linear image formingelement includes a glass lens. A material cost is high for the glasslens, and a processing cost is increased because the processes performedby a machine, such as a cutting and polishing are involved. However, adegree of a change of the optical axis of the glass lens in thesub-scanning direction caused by a temperature fluctuation is notsignificant because a linear expansion coefficient of a glass ismaintained within a range of about 5.0E-06 (1/° C.) and 10.0E-06 (1/°C.). When a lens made of a resin is used as the linear image formingelement, a cost for a material is kept low and a processing cost isdecreased because the processes are performed easily, resulting in areduction in costs. However, the degree of the change of the opticalaxis of the resin lens in the sub-scanning direction caused by thetemperature fluctuation becomes significant because the linear expansioncoefficient becomes high, i.e., about 7.0E-05 (1/° C.).

In a structure of fixing a lens illustrated in FIGS. 3 to 5, the opticalaxis of the lens changes according to a expansion or a shrinkage of theresin imaging lens 3 caused by the temperature fluctuation. Thus, adegree of a change of a scanning position on a surface of aphotoconductive element in the sub-scanning direction becomessignificant. A resin including aluminum or glass fiber is used as amaterial for the housing 14. These materials have a linear expansioncoefficient of 2.0E-05 (1/° C.) which is higher than that of a glass. Inthe lens fixing structure illustrated in FIG. 6, the scanning positionsignificantly changes in the sub-scanning direction according to aexpansion or a shrinkage of the housing 14 even if the resin imaginglens 3 is made of a glass.

FIG. 7 is a diagram illustrating a sectional view of a scanning opticalsystem in a sub-scanning direction showing a change of an optical axisdue to a temperature fluctuation. As illustrated in FIGS. 3 to 6, if theresin imaging lens 3 is fixedly provided directly to the housing 14before the deflector 4, the change of the scanning position in thesub-scanning direction becomes significant irrespective of the lensfixing structure.

As illustrated in FIG. 8, when an image forming lens includes the resinimaging lens 3 and a glass lens 16, the degree of the change of thescanning position in the sub-scanning direction caused by a temperaturefluctuation is reduced. However, because a liner expansion coefficientof a resin is higher than that of a glass, the change of the scanningposition in the sub-scanning direction due to the temperaturefluctuation is not prevented.

The present invention accommodates the change of the scanning positionin the sub-scanning direction since changes in the scanning positioncaused by temperature fluctuation are inevitable. According to thepresent invention, a direction of the change of the scanning position,which is caused by the temperature fluctuation, is aligned in aplurality of scanning optical systems. With this arrangement, a degreeof the change of the scanning position in the sub-scanning direction isreduced in the plurality of scanning optical systems.

FIG. 9 is a diagram illustrating a sectional view of the scanningoptical system in the sub-scanning direction after the deflector 4.Photoconductive elements 8A, 8A′, 8B, and 8B′ rotate in a clockwisedirection. In FIG. 9, a solid line indicates main rays of light in anordinary temperature while a chained line indicates main rays of lightwhen the temperature is fluctuated. In each scanning optical system,main rays of light in the fluctuated temperature is shifted in an upwarddirection in FIG. 9 with respect to main rays of light in the ordinarytemperature. In the scanning optical systems A and A′, the shifting ofmain rays of light in the fluctuated temperature results in the shiftingtoward the front side of the main rays of light in the normaltemperature with respect to the rotating direction of thephotoconductive elements 8A and 8A′.

In the scanning optical systems B and B′, the shifting of main rays oflight in the fluctuated temperature results in the shifting toward theback side of the main rays of light in the normal temperature withrespect to the rotating direction of the photoconductive elements 8B and8B′. In FIG. 9, a description of first and second scanning lenses 5A,5A′, 5B, 5B′, 6A, 6A′, 6B, and 6B′ are omitted for a purpose of asimplification.

The position of main rays of light in the fluctuated temperature on asurface of a photoconductive element is reversed with respect to theposition of main rays of light in the normal temperature, when anoptical path inflection mirror is employed. For example, in the scanningoptical system A, main rays of light in the fluctuated temperatureshifts toward the front side of main rays of light in the ordinarytemperature with respect to the rotating direction of thephotoconductive element 8A between the deflector 4 and the optical pathinflection mirror Ma1.

To the contrary, between the optical path inflection mirror Ma1 and thephotoconductive element 8A, main rays of light in the fluctuatedtemperature shifts toward the back side of main rays of light in theordinary temperature with respect to the rotating direction of thephotoconductive element 8A. The position of main rays of light in thefluctuated temperature is reversed by the optical path inflection mirrorMa1 with respect to main rays of light in the ordinary temperature. Theoptical path inflection mirror Ma1 shifts main rays of light in thefluctuated temperature toward the back side of main rays of light in theordinary temperature. with respect to the rotating direction of thephotoconductive element 8A.

As described above, the description of first and second scanning lenses5A, 5A′, 5B, 5B′, 6A, 6A′, 6B, and 6B′ are omitted for a purpose of asimplification in FIG. 9. In addition, although main rays of light inthe fluctuated temperature is indicated to shift parallel to main raysof light in the ordinary temperature, main rays of light in thefluctuated temperature may not always shift parallel to main rays oflight in the ordinary temperature, and a gradient of main rays of lightin the fluctuated temperature in FIG. 9 may vary according to afluctuation of a temperature. However, even if main rays of light in thefluctuated temperature does not shift parallel to main rays of light inthe ordinary temperature, an optical path inflection mirror shifts mainrays of light in the fluctuated temperature toward the back side of mainrays of light in the ordinary temperature with respect to the rotatingdirection of the photoconductive element 8A.

A direction of a shifting of a scanning position in a sub-scanningdirection caused by a temperature fluctuation in a set of scanningoptical system provided at both sides of the deflector 4 is describedbelow. For example, in a set of the scanning optical systems A and B′,in the scanning optical system A, main rays of light in the fluctuatedtemperature shifts toward the front side of main rays of light in theordinary temperature with respect to the rotating direction of thephotoconductive element 8A between the deflector 4 and the optical pathinflection mirror Ma1.

In the scanning optical system B′, main rays of light in the fluctuatedtemperature shifts toward the back side of main rays of light in theordinary temperature with respect to the rotating direction of thephotoconductive element 8B′ between the deflector 4 and the optical pathinflection mirror Mb′1. The following expression should be satisfied toalign a direction of change of a scanning position in a sub-scanningdirection caused by a temperature fluctuation in scanning opticalsystems A and B′:|N−M|=2k+1   (1)In expression (1), the number of optical path inflection mirrorsprovided in scanning optical systems B′ and A is respectivelyrepresented by “N” (i.e., N≧2) and M (i.e., M≧1), for “k” being aninteger equal to 0 or larger.

The number of optical path inflection mirrors provided in the scanningoptical systems A and B′ is one piece and two pieces, respectively. Assuch, the above-described expression is satisfied. Hence, the directionof change of the scanning position in the sub-scanning direction causedby the temperature fluctuation aligns in scanning optical systems A andB′.

As described above, in an optical scanning device having a plurality ofscanning optical systems A and B′, an amount of change in a relativescanning position of each scanning optical system is decreased. Therespective scanning optical systems A and B′ include the semiconductorlaser 1A or 1B′, the deflector 4, scanning lenses 5A and 6A or 5B′ and6B′, optical path inflection mirrors Ma1 or Mb′l and Mb′2, and imaginglenses 3A or 3B′. The scanning lenses 5A, 6A, 5B′, and 6B′ condense raysof scanning light to a surface of a photoconductive element. The imaginglenses 3A and 3B′ lead rays of light emitted from the semiconductorlaser 1A or 1B′ to the deflector 4. The deflector 4 scans rays of lightemitted from the semiconductor laser 1A or 1B′ and is commonly used inthe scanning optical systems A and B′. The scanning optical systems Aand B′ are arranged as a set at both sides of the deflector 4 having thedeflector 4 therebetween such that respective light fluxes scanned bythe deflector 4 become approximately parallel in a main scanningdirection.

If the number of optical path inflection mirrors in the scanning opticalsystems B′ and A (i.e., “N” and “M”, respectively) is arranged tosatisfy the expression of: |N−M|=2k+1 (i.e., N≧2, M≧1, and “k” is aninteger equal to 0 or larger), a direction of change of a scanningposition in a sub-scanning direction caused by a temperature fluctuationaligns in scanning optical systems A and B′. This is true even if anoptical axis of the imaging lenses 3A and 3B′ changes due to thetemperature fluctuation. Thus, an amount of change in a relativescanning position of each scanning optical system is decreased. Further,because the deflector 4 is commonly used in the scanning optical systemsA and B′, the number of parts used and a consumption of an electricpower are decreased.

Another example of a set of the scanning optical systems (i.e., A and B,A′and B, and A′ and B′), which are arranged as the set at both sides ofthe deflector 4 having the deflector 4 therebetween, are describedbelow. The number of optical path inflection mirrors provided in thescanning optical systems A and B is one piece and two pieces,respectively that satisfies the above-described expression (1). Thenumber of optical path inflection mirrors provided in the scanningoptical systems A′ and B is three pieces and two pieces, respectivelythus satisfying expression (1) described above. In the scanning opticalsystems A′ and B′, three and two pieces of optical path inflectionmirrors are provided, respectively that satisfies the above-describedexpression (1). Thus, a direction of change of a scanning position in asub-scanning direction caused by a temperature fluctuation aligns inrespective sets of the scanning optical systems (i.e., A and B, A′ andB, and A′ and B′).

As described above, if the number of optical path inflection mirrorsprovided in each set of the scanning optical systems satisfies theexpression (1), the direction of change of the scanning position in thesub-scanning direction caused by the temperature fluctuation aligns ineach set of the scanning optical systems. Thus, an optical scanningdevice, in which an amount of change in a relative scanning position ofeach scanning optical system is small, is obtained.

Next, a direction of change of a scanning position in a sub-scanningdirection caused by a temperature fluctuation in the scanning opticalsystems that are arranged in the sub-scanning direction at both sides ofthe deflector 4 is described below. For example, in the scanning opticalsystems A and A′, main rays of light in the fluctuated temperatureshifts toward the front side of main rays of light in the ordinarytemperature with respect to the rotating direction of thephotoconductive element 8A between the deflector 4 and the optical pathinflection mirror Ma1 in the scanning optical system A. In the scanningoptical system A′, main rays of light in the fluctuated temperatureshifts toward the front side of main rays of light in the ordinarytemperature with respect to the rotating direction of thephotoconductive element 8A′ between the deflector 4 and the optical pathinflection mirror Ma′1.

Thus, the number of optical path inflection mirrors in the scanningoptical systems A and A′ is arranged to satisfy the expression describedbelow in order to align a direction of change of a scanning position ina sub-scanning direction caused by a temperature fluctuation, for anumber of optical path inflection mirrors provided in the scanningoptical systems A′ and A being respectively represented by “Q” (i.e.,Q≧2) and “P” (i.e., P≧1), and where “k” is an integer equal to 0 orlarger.|Q−P|=2k   (2)The expression (2) indicates that a difference of the number of opticalpath inflection mirrors provided in each scanning optical systemarranged in the sub-scanning direction becomes zero or an even number.

In the scanning optical systems A and A′, one piece and three pieces ofoptical path inflection mirrors are provided, respectively, therebysatisfying the expression (2). Thus, the direction of change of thescanning position in the sub-scanning direction caused by thetemperature fluctuation in the scanning optical systems A and A′ isaligned. Two pieces of the optical path inflection mirrors are providedin each of the scanning optical systems B and B′, thereby satisfying theexpression (2). Hence, the direction of change of the scanning positionin the sub-scanning direction caused by the temperature fluctuation inthe scanning optical systems B and B′ is aligned.

According to the example of the present invention as described above,the direction of change of the scanning position in the sub-scanningdirection caused by the temperature fluctuation aligns in the scanningoptical systems A and A′ even if an optical axis of the imaging lenses3A and 3A′ changes due to the temperature fluctuation, when thedifference of the number of optical path inflection mirrors in thescanning optical systems A and A′ is equal to 0 or an even number. Thus,an amount of change in a relative scanning position of each scanningoptical system is decreased.

In the example illustrated in FIG. 9, two scanning optical systems arearranged in the sub-scanning direction at both sides of the deflector 4.However, the number of scanning optical systems is not limited to thisexample. Namely, three or more numbers of scanning optical systems maybe arranged. For example, when three scanning optical systems A, A′, andA″ are provided in the sub-scanning direction, the expression (2) needsto be satisfied in each set of scanning optical systems (i.e., A and A′,A′ and A″, and A and A″).

In addition, the number of scanning optical systems in the sub-scanningdirection may not be same in both sides of the deflector 4. Namely, whenthe number of scanning optical systems in the sub-scanning directionprovided at one side of the deflector 4 represents “S” (i.e., S≧2) whilethe number of scanning optical systems in the sub-scanning directionprovided at the other side of the deflector 4 represents “T” (i.e.,T≧2). The “S” and “T” are not always required to be equal in number(i.e., S≠T). In the example illustrated in FIG. 9, the “S” and “T” areequal in number (i.e., two).

Next, another example of the present invention is described below. Inthe example illustrated in FIG. 1, the scanning optical systems A and Bare provided at a position opposed to each other having the deflector 4therebetween, and the scanning optical systems A and A′ are provided ina sub-scanning direction. In this example, a plurality of scanningoptical systems are arranged only in the sub-scanning direction. Namely,the scanning optical systems A and A′ are provided but the scanningoptical systems B and B′ in FIG. 1 are not provided.

Components other than those described above are arranged similar tothose in the above-described example. In this example, when theexpression (2) is satisfied, a direction of change of a scanningposition in a sub-scanning direction caused by a temperature fluctuationaligns. The number of scanning optical systems arranged in thesub-scanning direction is not limited to two, and may be equal to threeor more. In this case, the expression (2) needs to be satisfied in anarbitary two scanning optical systems.

Namely, in an optical scanning device having a plurality of scanningoptical systems A and A′, an amount of change in a relative scanningposition of each scanning optical system is decreased. The respectivescanning optical systems A and A′ include the semiconductor laser 1A or1A′,the deflector 4, scanning lenses 5A and 6A or 5A′ and 6A′, opticalpath inflection mirrors Ma1 or Ma′1, Ma′2 and Ma′3, and imaging lenses3A or 3A′. The scanning lenses 5A, 6A, 5A′, and 6A′ condense rays ofscanning light to a surface of a photoconductive element. The imaginglenses 3A and 3A′ lead rays of light emitted from the semiconductorlaser 1A or 1A′ to the deflector 4. The deflector 4 scans rays of lightemitted from the semiconductor laser 1A or 1A′ and is commonly used inthe scanning optical systems A and A′. Because the difference of thenumber of the optical path inflection mirrors in the scanning opticalsystems A and A′ is equal to 0 or an even number, a direction of changeof a scanning position in a sub-scanning direction caused by atemperature fluctuation aligns in scanning optical systems A and A′ evenif an optical axis of the imaging lenses 3A and 3A′ changes due to thetemperature fluctuation.

In the optical scanning devices described above, a resin lens is used asthe imaging lens 3. Thus, the material costs for resin and the cost ofprocessing the resin materials are decreased, as compared to the costsassociated with a glass lens. Next, a construction of an image formingapparatus having the above-described optical scanning device isdescribed below.

FIG. 10 is a schematic drawing illustrating a construction of a tandemimage forming apparatus. An image forming apparatus 100 includes anoptical scanning device 117, a transfer belt 114, and a fixing device116. Above the transfer belt 114, photoconductive elements 111M, 111C,111Y, and 111K for magenta (M), yellow (Y), cyan (C), and black (K)toner, respectively are arranged in order from an upstream to adownstream in a moving direction of the transfer belt 114.

Around the photoconductive element 111M, a charging device 112M, adeveloping device 113M, and a transfer device 114M are arranged toperform an electrophotographic printing process. A similar devices arearranged around the other photoconductive elements 111C, 111Y, and 111K.In the tandem image forming apparatus, when a printing mode using aplurality of colors is selected, each photoconductive element 111M,111C, 111Y, and 111K is exposed by an exposure unit (not shown) based onan image signal corresponding to each color to form an electrostaticlatent image on a surface of each photoconductive element 111M, 111C,111Y, and 111K. Each electrostatic latent image is developed into atoner image with toner of respective colors. Each color toner image iselectrostatically attracted onto the transfer belt 114. Each color tonerimage is then transferred onto a transfer sheet P one after another sothat each color toner image is superimposed on each other.

The toner images transferred onto the transfer sheet P are then fixed.Then, the transfer sheet P having a multicolor image is discharged. Whena single color printing mode is selected, photoconductive elements andrelated devices used for colors other than the selected color are putinto a non-operating state. An electrostatic latent image is formed on asurface of a photoconductive element of the selected color. Theelectrostatic latent image is then developed into a toner image withtoner of the selected color. The toner image is electrostaticallyattracted onto the transfer belt 114. The toner image is thentransferred onto the transfer sheet P. The toner image transferred ontothe transfer sheet P is then fixed. Then, the transfer sheet P having asingle color image is discharged.

If the above-described optical scanning device is used as the opticalscanning device 117 of the image forming apparatus 100, a direction ofchange of a scanning position in a sub-scanning direction on aphotoconductive element corresponding to each color aligns even if anoptical axis of a resin imaging lens in the optical scanning device 117shifts to the sub-scanning direction due to a temperature fluctuation.

Thus, in the image forming apparatus 100, an amount of change in arelative scanning position of each scanning optical system is decreased,resulting in a production of a high quality image while obviating acreation of a color shift.

Obviously, numerous additional modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the present invention may be practiced otherwise than as specificallydescribed herein.

This document claims priority and contains subject matter related toJapanese Patent Application No. 2000-402660, filed on Dec. 28, 2000, andthe entire contents thereof are herein incorporated by reference.

1. An optical scanning device, comprising: a plurality of scanningoptical systems configured to scan different scanning surfaces, each ofthe scanning optical systems comprising: a light source configured toemit a light flux; a deflector configured to scan the light flux emittedfrom the light source, wherein the deflector is commonly used in theplurality of scanning optical systems; a scanning lens configured tocondense the scanned light flux to the scanning surface; an optical pathinflection mirror configured to inflect the scanned light flux; and animaging lens configured to lead the light flux emitted from the lightsource to the deflector, wherein the plurality of scanning opticalsystems are provided at both sides of the deflector with the deflectorbeing therebetween such that one each of the scanning optical systems atboth sides of the deflector comprise a set of the optical scanningsystem and respective light fluxes scanned by the deflector, the set ofthe optical scanning system being approximately parallel in a mainscanning direction, and wherein an expression,|N−M|=2k+1 is satisfied when a number of optical path inflection mirrorsprovided in each of the set of scanning optical systems is representedby “N” where N≧2, “M” where M≧1 and where “k” is an integer equal tozero or larger.
 2. The optical scanning device according to claim 1,wherein one or more of the plurality of scanning optical systems areprovided in a sub-scanning direction at both sides of the deflector inaddition to the scanning optical systems provided at both sides of thedeflector with the deflector being therebetween.
 3. The optical scanningdevice according to claim 2, wherein a difference of the number ofoptical path inflection mirrors between two of the plurality of scanningoptical systems in the sub-scanning direction at both sides of thedeflector is set to zero or an even number.
 4. (canceled)
 5. The opticalscanning device according to claim 1, wherein the scanning opticalsystem comprises a plurality of scanning lenses and an optical pathinflection mirror being configured among the plurality of scanninglenses.
 6. (canceled)
 7. The optical scanning device according to claim1, wherein the imaging lens includes a resin lens.
 8. (canceled)
 9. Theoptical scanning device according to claim 7, further comprising: ahousing to which the light source and the imaging lens are provided. 10.(canceled)
 11. The optical scanning device according to claim 9, whereinthe imaging lens is directly affixed to the housing.
 12. (canceled) 13.An image forming apparatus, comprising: a transfer sheet feeding device;and an optical scanning device including a plurality of scanning opticalsystems configured to scan different scanning surfaces, each of thescanning optical systems comprising: a light source configured to emit alight flux; a deflector configured to scan the light flux emitted fromthe light source, wherein the deflector is commonly used in theplurality of scanning optical systems; a scanning lens configured tocondense the scanned light flux to the scanning surface; an optical pathinflection mirror configured to inflect the scanned light flux; and animaging lens configured to lead the light flux emitted from the lightsource to the deflector, wherein the plurality of scanning opticalsystems are provided at both sides of the deflector with the deflectortherebetween such that one each of the scanning optical systems at bothsides of the deflector comprise a set of the optical scanning system andrespective light fluxes scanned by the deflector, the set of the opticalscanning system being approximately parallel in a main scanningdirection, and wherein an expression, is satisfied when a number ofoptical path inflection mirrors provided in each of the set of scanningoptical systems is represented by “N” where N≧2, “M” where M≧1, andwhere “k” is an integer equal to zero or larger.
 14. (canceled)
 15. Anoptical scanning device, comprising: a plurality of scanning opticalsystems configured to scan different scanning surfaces, each of thescanning optical systems comprising: a light source means for emitting alight flux; a deflector means for scanning the light flux emitted fromthe light source means, wherein the deflector means is commonly used inthe plurality of scanning optical systems; a scanning lens means forcondensing the scanned light flux to the scanning surface; an opticalpath inflection mirror means for inflecting the scanned light flux; andan imaging lens means for leading the light flux emitted from the lightsource means to the deflector means, wherein the plurality of scanningoptical systems are provided at both sides of the deflector means withthe deflector means being therebetween such that one each of thescanning optical systems at both sides of the deflector means comprise aset of the optical scanning system and respective light fluxes scannedby the deflector means in the set of the optical scanning system becomeapproximately parallel in a main scanning direction, and wherein anexpression,|N−M|=2k+1 is satisfied when a number of optical path inflection mirrormeans provided in each of the set of scanning optical systems isrepresented by “N” where N≧2, “M” where M≧1, and where “k” is an integerequal to zero or larger.
 16. The optical scanning device according toclaim 15, wherein one or more of the plurality of scanning opticalsystems are provided in a sub-scanning direction at both sides of thedeflector means in addition to the scanning optical systems provided atboth sides of the deflector means with the deflector means beingtherebetween.
 17. The optical scanning device according to claim 16,wherein a difference of the number of optical path inflection mirrormeans between two of the plurality of scanning optical systems in thesub-scanning direction at both sides of the deflector means is set tozero or an even number.
 18. (canceled)
 19. The optical scanning deviceaccording to claim 15, wherein the scanning lens means is one of aplurality of scanning lens means and an optical path inflection mirrormeans is provided among the plurality of scanning lens means. 20.(canceled)
 21. The optical scanning device according to claim 15,wherein the imaging lens means comprises a resin lens.
 22. (canceled)23. The optical scanning device according to claim 21, furthercomprising: a housing means configured to support the light source meansand the imaging lens means.
 24. (canceled)
 25. The optical scanningdevice according to claim 23, wherein the imaging lens means is directlyaffixed to the housing means.
 26. (canceled)
 27. An image formingapparatus, comprising: a transfer sheet feeding means for feeding atransfer sheet; and an optical scanning means including a plurality ofscanning optical systems for scanning different scanning surfaces, eachof the scanning optical systems comprising: a light source means foremitting a light flux; a deflector means for scanning the light fluxemitted from the light source means, wherein the deflector means iscommonly used in the plurality of scanning optical systems; a scanninglens means for condensing the scanned light flux to the scanningsurface; an optical path inflection mirror means for inflecting thescanned light flux; and an imaging lens means for leading the light fluxemitted from the light source means to the deflector means, wherein theplurality of scanning optical systems are provided at both sides of thedeflector means with the deflector means being therebetween such thatone each of the scanning optical systems at both sides of the deflectormeans comprise a set of the optical scanning system and respective lightfluxes scanned by the deflector means, in the set of the opticalscanning system being approximately parallel in a main scanningdirection, and wherein an expression,|N−M|=2k+1 is satisfied when a number of optical path inflection mirrormeans provided in each of the set of scanning optical systems isrepresented by “N” where N≧2, 37 M” where M≧1, and where “k” is aninteger equal to zero or larger.
 28. (canceled)
 29. A method fordecreasing an amount of change in a relative scanning position, themethod comprising: providing a plurality of scanning optical systemsconfigured to scan different scanning surfaces, each of the scanningoptical systems comprising: a light source to emit a light flux; adeflector to scan the light flux emitted from the light source, whereinthe deflector is commonly used in the plurality of scanning opticalsystems; a scanning lens to condense the scanned light flux to thescanning surface; an optical path inflection mirror to inflect thescanned light flux; and an imaging lens to lead the light flux emittedfrom the light source to the deflector, wherein the plurality ofscanning optical systems are provided at both sides of the deflectorwith the deflector being therebetween such that one each of the scanningoptical systems at both sides of the deflector comprise a set of theoptical scanning system and respective light fluxes scanned by thedeflector, the set of the optical scanning system being approximatelyparallel in a main scanning direction, and wherein an expression,|N−M|=2k+1 is satisfied when the number of optical path inflectionmirrors provided in each of the set of scanning optical systems isrepresented by “N” where N≧2, “M” where M≧1, and where “k” is an integerequal to zero or larger.
 30. (canceled)